analyteþH 2 þO 2 !combustion productsþH 2 Oþionsþradicalsþelectrons
X
ðionsÞþ
X
ðelectronsÞ!current
- Nitrogen–phosphorus detector(NPD) (also called athermionic detector): This is
similar in design to an FID but has a crystal of a sodium salt fused onto the electrode
system, or a burner tip embedded in a ceramic tube containing a sodium salt or a
rubidium chloride tip. The NPD has excellent selectivity towards nitrogen- and
phosphorus-containing analytes and shows a poor response to analytes possessing
neither of these two elements. Its linearity (10^5 ) and upper temperature limit (300C)
are not quite as good as an FID but its detection limit (10–14gs–1) is better. It is widely
used in organophosphorus pesticide residue analysis. - Electron capture detector(ECD): This responds only to analytes that capture
electrons, particularly halogen-containing compounds. This detector is widely used
in the analysis of polychlorinated compounds, such as the pesticides DDT, dieldrin
and aldrin. It has a very high sensitivity (10–13gs–1) and an upper temperature limit
of 300C but its linear range (10^2 to 10^4 ) is much lower than that of the FID. The
detector works by means of a radioactive source (^63 Ni) ionising the carrier gas and
releasing an electron that gives a current across the electrodes when a suitable
voltage is applied. When an electron-capturing analyte (generally one containing
a halogen atom) emerges from the column, the ionised electrons are captured, the
current drops and this change in current is recorded. The carrier gas most commonly
used in conjunction with an ECD is nitrogen or an argonþ5% methane mixture. - Flame photometric detector: This exploits the fact the P- and S-containing analytes
emit light when they are burned in a FID-type detector. This light is detected and
quantified. The detection limit is of the order of 1.0 pg for P-containing compounds
and 20 pg for S-containing compounds. - Rapid scanning Fourier transform infrared detector: This records the infrared spectrum
of the emerging analytes and can give structural as well as quantitative information
about the analyte. Any analyte with an infrared spectrum can be detected with a
detection limit of about 1 ng.
Collector
electrode
Air
supply
Hydrogen
supply
Carrier
gas
Flame
Outlet
Amplifier
Igniter
Fig. 11.13GC flame ionisation detector. The tip of the flame forms the anode and the collector
electrode the cathode.
475 11.9 Gas chromatography